Ferroelectric based capacitors are in increasing demand as integrated circuit elements. Capacitors having lead lanthanum titanium zirconate (PLZT) dielectrics offer large dielectric constants which in turn make the construction of small capacitors with relatively large capacitances possible. A ferroelectric capacitor consists of a PLZT layer sandwiched between two planar electrodes. Capacitors utilizing platinum electrodes are particularly advantageous, since Pt electrodes can withstand the high processing temperatures inherent in PLZT based capacitor fabrication while providing an electrode structure having high conductivity.
The interaction of the platinum electrode with the PLZT produces a Schottky diode at the interface of the PLZT and platinum electrode. In applications in which information is stored in the ferroelectric by altering the polarization of the ferroelectric, the voltage difference that must be applied across the capacitor to switch the polarization is increased by the presence of this diode since the voltage drop needed to forward bias the diode is a significant fraction of the potential difference needed to switch the polarization of the ferroelectric.
The diode in question can be eliminated if an ohmic material is used as the electrode in contact with the PLZT. Unfortunately, such ohmic materials have relatively high resistivities, and hence, must be connected to a higher conductivity structure that provides the connection to the power source. One common method for constructing an ohmic contact with sufficient conductivity is to sandwich the ohmic material between a platinum electrode and the ferroelectric layer. Platinum is preferred for this purpose because it can withstand the high processing temperatures encountered in the subsequent fabrication steps. A particularly useful ohmic material is lanthanum strontium cobalt oxide (LSCO).
The bottom electrode of a PLZT capacitor is typically deposited on the semiconductor surface or a SiO.sub.2 layer thereon. To "stick"the platinum layer to the silicon, a titanium layer is deposited on the silicon surface prior to depositing the platinum layer. The LSCO layer is then deposited in order to isolate the platinum electrode from the PLZT to prevent the formation of the diode junction described above. The LSCO is typically deposited by means of sputtering at high temperatures, typically 600.degree. C., in order to properly crystalize the LSCO layer. A second layer of LSCO is then deposited after the PLZT material is deposited and annealed. Finally, this top LSCO layer is etched to form the top electrode of the capacitor.
Despite the benefits of the LSCO layer, there are disadvantages inherent in the above-described method of depositing LSCO and the etching thereof. High temperatures are necessary during the LSCO deposition process in order to adequately crystalize the LSCO on the platinum. At such high temperatures, the presence of oxygen around the titanium-platinum junction causes the titanium to oxidize, impeding crystalization of the LSCO layer. These conditions also cause the platinum layer to buckle or "grow hillocks." Consequently, when the LSCO layer is etched, there are thin spots between the platinum electrodes which can eventually lead to shorts between them.
Another disadvantage of prior art fabrication of the LSCO layer, is the need to use a very expensive ion beam etching method. There are no effective chemical etching processes that can be used to shape the LSCO layer. The high temperatures used during the deposition of the LSCO foreclose lift-off methods. Hence, the LSCO electrodes must be formed by ion beam etching of an LSCO layer.
Yet another disadvantage of prior art methods for fabricating the LSCO layer, is the likelihood of misalignment between the LSCO and platinum layers. This in turn leads to under-or oversizing the memory cell.
Accordingly, it is the general object of the present invention to provide a method for making an improved LSCO electrode stack.
It is a further object of the present invention to provide a method for making an LSCO layer which does not require the LSCO to be deposited at high temperatures.
It is also an object of the present invention to provide a method for making an LSCO layer which does not require ion beam etching.
Yet another object of the present invention is to provide a method for making an LSCO layer in which the LSCO and electrode layers are self-aligning.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.